1. Field of the Invention
The present invention relates generally to a channel estimation apparatus and method in an OFDM (Orthogonal Frequency Division Multiplexing) communication system, and in particular, to an apparatus and method for performing accurate channel estimation by canceling inter-cellular interference in a MIMO (Multiple Input Multiple Output)-OFDM communication system.
2. Description of the Related Art
Typically, a wireless communication system refers to a system supporting wireless communication service, which includes Node Bs and UEs (User Equipment). The Node B and the UE support the wireless communication service in transmission frames. For frame transmission and reception, therefore, synchronization must be acquired between a Node B and a UE. Accordingly, the Node B transmits a synchronization signal to the UE, such that the UE can identify the start of a frame. The UE then detects the frame timing of the Node B from the synchronization signal and demodulates a received frame based on the frame timing. In general, the synchronization signal is a preamble sequence preset between the Node B and the UE.
Preferably, a multi-carrier OFDM communication system uses a preamble sequence having a low PAPR (Peak-to-Average Power Ratio). The Node B transmits to the UE the first part of a long preamble for coarse synchronization, followed by a short preamble for fine synchronization. The UE transmits only a short preamble to the Node B, for fine synchronization.
The OFDM communication system transmits user data to a plurality of users, i.e., UEs, by multiplexing a frame in time. Simultaneously, a frame preamble is transmitted for a predetermined time period starting from the start of a frame, to indicate the start of the frame. Because of burst data transmission to users in one frame, a burst preamble exists in front of each user data in order to indicate the start of the data. Therefore, the UE receives the data preamble to determine the start of its user data. More specifically, to synchronize it's timing to the start of data for data reception, the UE receives a common preamble sequence in the system and acquires synchronization, prior to signal reception.
The OFDM communication system uses the same source coding, channel coding, and modulation as non-OFDM communication systems. Compared to a CDMA (Code Division Multiple Access) communication system in which data is spread prior to transmission, the OFDM communication system inserts a guard interval into an IFFT (Inverse Fast Fourier Transform) signal. Therefore, the OFDM communication system can transmit a broadband signal with simple hardware relative to the CDMA communication system. The OFDM communication system IFFT-processes a modulated bit-symbol sequence, thereby producing a time-domain signal. The time-domain signal (i.e. OFDM symbol) is a broadband signal in which a plurality of narrow-band subcarrier signals are multiplexed. A plurality of modulated symbols are delivered for one OFDM symbol period.
However, simple transmission of an IFFT OFDM symbol without any further processing leads to inevitable interference between the previous OFDM symbol and the present OFDM symbol. To cancel the ISI (Inter-Symbol Interference), a guard interval is inserted. It was proposed that null data is to be inserted for a predetermined interval as the guard interval. The distinctive shortcoming of this guard interval is that for an incorrect estimation of the start of the OFDM symbol at the receiver, interference occurs between subcarriers, increasing the wrong decision probability of the received OFDM symbol. Therefore, the guard interval is used in form of a “cyclic prefix” or “cyclic postfix”. The cyclic prefix is a copy of the last 1/n bits of a time-domain OFDM symbol, inserted into an effective OFDM symbol, and the cyclic postfix is a copy of the first 1/n bits of the time-domain OFDM symbol, inserted into the effective OFDM symbol. Utilizing the guard interval as the redundant information of the copied first or last part of one OFDM symbol, the receiver can acquire the time/frequency synchronization of a received OFDM symbol.
A signal transmitted from the transmitter is distorted as it experiences a radio channel and thus the distorted signal arrives at the receiver. The receiver performs channel estimation by acquiring time/frequency synchronization using a known preamble sequence, and channel-compensates frequency-domain FFT (Fast Fourier Transform) symbols using the channel estimate. The receiver then recovers information data by channel decoding and source decoding the channel-compensated symbols in correspondence with the channel coding and source coding used in the transmitter.
The OFDM communication system uses a preamble sequence to achieve frame timing synchronization, frequency synchronization, and channel estimation. Although a guard interval and pilot subcarriers can be used instead of the preamble in frame timing synchronization, frequency synchronization, and channel estimation, the transmitter usually transmits known symbols at the start of every frame or data burst as a preamble sequence and the receiver updates time/frequency/channel information with the preamble sequence.
The importance of channel estimation lies in coherent modulation and demodulation in the OFDM system. A channel estimator is a required for systems using coherent modulation and demodulation. Especially under a MIMO environment, channel information is needed for every antenna, further increasing the importance of the channel estimation.
When the MIMO-OFDM system supports a cellular environment, severe interference occurs at cell boundaries, thereby degrading channel estimation performance. Accordingly, a need exists for channel estimation techniques that minimize inter-cellular interference in the MIMO-OFDM cellular system.